Human medullary thyroid cancer (MTC) is a neuroendocrine cancer that either arises sporadically or as an inherited neoplasm from the calcitonin producing C-cells of the thyroid. These cells arise in the neural crest and were among the first characterized to have "APUD" (amine precursor uptake and decarboxylation) features characteristic of neuroendocrine cells. Human MTC can have an extremely variable clinical course, with some patients having chronic, relatively indolent disease and others dying rapidly from virulent tumors. Previous studies in the literature and our preliminary studies have used a stable human MTC cell line, the TT line, to study the in vitro differentiation of MTC. In the present studies the in vitro differentiation of MTC will be further examined followed by the use of a differential screening strategy to clone potential differentiation associated markers or genes. In addition, because of the ready availability of clinical material at the Mayo Clinic, the potential clinical significance of the Cloned genes will be examined. In this phase of the studies, the expression in human MTCs of the differentiation associated genes identified earlier will be analyzed against a series of clinical parameters to see if variation in expression of these gene has any clinical correlations. By thus combining the in vitro studies with the analysis of human tumors in vivo, the proposed studies may eventually lead to refined prognostic criteria for human MTC. The improved identification of patients with aggressive disease would then help target them for closer follow-up and/or more intensive therapy. In addition, these studies may lead to a better understanding of the mechanisms modulating the differentiation status of MTC.